To prepare your 2018 tax return, you will need to Download TurboTax for 2018 and mail in your return. 

Click this link for more info on How to Prepare a Prior Year Return . 

You cannot file a previous years return with the Online version, the Online version is only to file the current years tax return.

Prior year returns cannot be e-filed with TurboTax.  You file your 2018 return with TurboTax you will need to use the 2018 Download software (or go to the IRS website for prior year forms).  To purchase the software from TurboTax go to the following website.

https://turbotax.intuit.com/personal-taxes/past-years-products/